Aluminium is produced conventionally by the Hall-Heroult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures up to around 950.degree. C. A Hall-Heroult reduction cell typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate forming the cell bottom floor. The cathode substrate is usually a carbon lining made of prebaked anthracite-graphite or all graphite cathode blocks, joined with a ramming mixture of anthracite, coke, and coal tar.
In Hall-Heroult cells, a molten aluminium pool acts as the cathode. The carbon lining or cathode material has a useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminium as well as intercalation of sodium, which causes swelling and deformation of the cathode carbon blocks and ramming mix. In addition, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides.
Difficulties in operation also arise from the accumulation of undissolved alumina sludge on the surface of the carbon cathode beneath the aluminium pool which forms insulating regions on the cell bottom. Penetration of cryolite and aluminium through the carbon body and the deformation of the cathode carbon blocks also cause displacement of such cathode blocks. Due to cracks in the cathode blocks, aluminium reaches the steel cathode conductor bars causing corrosion thereof leading to deterioration of the electrical contact, non uniformity in current distribution and an excessive iron content in the aluminium metal produced.
A major drawback of carbon as cathode material is that it is not wetted by aluminium. This necessitates maintaining a deep pool of aluminium (100-250 mm thick) in order to ensure a certain protection of the carbon blocks and an effective contact over the cathode surface. But electromagnetic forces create waves in the molten aluminium and, to avoid short-circuiting with the anode, the anode-to-cathode distance (ACD) must be kept at a safe minimum value, usually 40 to 60 mm. For conventional cells, there is a minimum ACD below which the current efficiency drops drastically, due to short-circuiting between the aluminium pool and the anode or to oxidation of the aluminium. The electrical resistance of the electrolyte in the inter-electrode gap causes a voltage drop from 1.8 to 2.7 volts, which represents from 40 to 60 percent of the total voltage drop, and is the largest single component of the voltage in a given cell.
To reduce the ACD and associated voltage drop, extensive research has been carried out with Refractory Hard Metals or Refractory Hard Materials (RHM) such as TiB.sub.2 as cathode materials. TiB.sub.2 and other RHM's are practically insoluble in aluminium, have a low electrical resistance, and are wetted by aluminium. This should allow aluminium to be electrolytically deposited directly on an RHM cathode surface, and should avoid the necessity for a deep aluminium pool. Because titanium diboride and similar Refractory Hard Metals are wettable by aluminium, resistant to the corrosive environment of an aluminium production cell, and are good electrical conductors, numerous cell designs utilizing Refractory Hard Metal have been proposed, which would present, many advantages, notably including the saving of energy by reducing the ACD.
The use of titanium diboride and other RHM current-conducting elements in electrolytic aluminium production cells is described inter alia in U.S. Pat. Nos. 2,915,442, 3,028,324, 3,215,615, 3,314,876, 3,330,756, 3,156,639, 3,274,093 and 3,400,061. Despite extensive efforts and the potential advantages of having surfaces of titanium diboride at the cell cathode bottom, such propositions have not been commercially adopted by the aluminium industry.
U.S. Pat. No. 4,544,457 (Sane) discloses a drained cathode for an aluminium production cell having an apertured sheath of corrosion-resistant material which closely conforms to the cathode surface and retains molten aluminium in stagnant contact with the cathode surface.
U.S. Pat. No. 3,028,324 (Ransley) proposed to immerse titanium diboride structures in molten aluminium and reduce the dissolution of titanium diboride by maintaining a concentration of titanium and boron in the molten aluminium.
U.S. Pat. No. 4,560,448 (Sane) proposed coating refractory non-carbon bodies with a thin coating of titanium diboride which was maintained, when the bodies were immersed in a cathodic aluminium pool of an aluminium electrowinning cell, by maintaining a concentration of titanium and boron in the molten aluminium sufficient to inhibit dissolution of the titanium diboride.
However, this principle has not been applied successfully to carbon cathodes coated with refractory hard metal borides.
To avoid the problems encountered with carbon cathodes coated with refractory hard metal borides, U.S. Pat. No. 5,227,045 (Townsend) proposed a development of the above idea where a drained carbon cathode having a titanium diboride coating in a carbon binder was protected by maintaining a supersaturated concentration of titanium and boron in the molten aluminium film sufficient to deposit a protective titanium diboride coating at a rate of about 0.01 to 2 cm per year.
This U.S. Pat. No. 5,227,045 examines the effects of operation with differing titanium and boron levels in the molten aluminium film and shows that, below saturation of titanium diboride in the molten aluminium, titanium diboride dissolves. Moreover, according to this patent, at below 200 ppm titanium there is a reaction between aluminium and carbon to form AlC, and TiC dissolves. At above 200 ppm titanium, dissolved titanium reacts with carbon and AlC to form TiC.
Also, it was found that, even just above the saturation of titanium diboride in the molten aluminium, at below 200 ppm Ti, there is still a reaction between aluminium and carbon to form AlC, which disrupts deposit of titanium diboride so it deposits too slow to form a protective coating. At above 200 ppm titanium, dissolved titanium reacts with carbon to form TiC, and titanium diboride still deposits tool slow to form a protective coating.
Thus, the carbon cathodes with titanium diboride/carbon coatings were found to be insufficient to resist dissolution and disintegration in the absence of a permanently grown protective titanium diboride deposit produced under constant supersaturation conditions. With levels of titanium and boron below or just above the saturation limit, the coatings were found to be unstable and could not be maintained for long periods.
Following this teaching therefore leads away from preventing dissolution of titanium diboride coatings on carbon-based cathodes by maintaining titanium and boron in the molten aluminium below a supersaturated condition at which titanium diboride permanently grows onto the surface.
Moreover, the production of aluminium for certain applications, for instance to make very thin aluminium foils, allows only a very low tolerance of boron, and the precipitation of titanium diboride crystals would be highly disadvantageous. For such applications, operating under supersaturation conditions according to U.S. Pat. No. 5,227,045 would be ruled out.